U.S. patent application number 11/118778 was filed with the patent office on 2006-07-20 for heat sink for surface-mounted semiconductor devices.
Invention is credited to Manuel Carmona, Georg Ernst, Markus Fink, Uwe Luckner.
Application Number | 20060158857 11/118778 |
Document ID | / |
Family ID | 36683657 |
Filed Date | 2006-07-20 |
United States Patent
Application |
20060158857 |
Kind Code |
A1 |
Luckner; Uwe ; et
al. |
July 20, 2006 |
Heat sink for surface-mounted semiconductor devices
Abstract
A heat sink is arranged on a main circuit board of an electronic
module. The heat sink includes a three-dimensionally structured
thermally conductive plate with a press-on region and with
snap-action hooks. The snap-action hooks are arranged approximately
at right angles with respect to the press-on region and are
resiliently connected to the press-on region. The snap-action hooks
are latched into place, with pressure generation of the press-on
region onto a rear side of a surface-mountable semiconductor
device, into corresponding passage openings of the circuit board. A
plastically deformable thermal composition is disposed between the
rear side of the semiconductor device and the press-on region of
the heat sink so as to form an intermediate layer therebetween to
provide compensation for the thickness tolerances of the
semiconductor device.
Inventors: |
Luckner; Uwe; (Bad Abbach,
DE) ; Ernst; Georg; (Thalmassing, DE) ;
Carmona; Manuel; (Barcelona, ES) ; Fink; Markus;
(Zell, DE) |
Correspondence
Address: |
EDELL, SHAPIRO & FINNAN, LLC
1901 RESEARCH BOULEVARD
SUITE 400
ROCKVILLE
MD
20850
US
|
Family ID: |
36683657 |
Appl. No.: |
11/118778 |
Filed: |
May 2, 2005 |
Current U.S.
Class: |
361/719 ;
257/E23.086; 257/E23.087; 257/E23.104 |
Current CPC
Class: |
H01L 2224/16 20130101;
H01L 2924/00014 20130101; H01L 2224/73253 20130101; H01L 23/4093
20130101; H01L 2924/00014 20130101; H01L 2224/0554 20130101; H01L
2224/0555 20130101; H01L 2224/0556 20130101; H01L 2224/73253
20130101; H01L 2224/05599 20130101; H01L 2224/0557 20130101; H01L
2224/05571 20130101; H01L 2924/01019 20130101; H01L 2924/15311
20130101; H01L 2924/01068 20130101; H01L 2924/16152 20130101; H01L
2924/00014 20130101; H01L 2924/00014 20130101; H01L 23/3675
20130101; H01L 2224/05573 20130101; H01L 2924/16152 20130101; H01L
2924/3025 20130101; H01L 23/42 20130101 |
Class at
Publication: |
361/719 |
International
Class: |
H05K 7/20 20060101
H05K007/20 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 20, 2005 |
DE |
10 2005 002 812.8 |
Claims
1. A heat sink for surface-mounted semiconductor devices on a main
circuit board of an electronic module including a surface-mounted
semiconductor device, the heat sink comprising a
three-dimensionally structured thermally conductive plate with a
press-on region and with snap-action hooks, the snap-action hooks
being resiliently connected to the press-on region of the heat sink
and being configured for latching, with pressure generation of the
press-on region onto a rear side of the surface-mounted
semiconductor device, into corresponding passage openings of the
circuit board, and a plastically deformable thermally conductive
composition disposed on the heat sink and being arranged to form an
intermediate layer between the rear side of the surface-mounted
semiconductor device and the press-on region of the heat sink.
2. The heat sink of claim 1, wherein a spring-elastic connection is
disposed between the press-on region and each of the snap-action
hooks such that the plastically deformable composition is capable
of elastic deformation upon application of a pressure applied in
the press-on region.
3. The heat sink of claim 1, wherein the heat sink further
comprises a thermally conductive copper alloy.
4. The heat sink of claim 3, wherein the conductive copper alloy
comprises 0.3% by weight of chromium, 0.1% by weight of titanium,
0.02% by weight of silicon and the remainder copper.
5. The heat sink of claim 1, wherein the heat sink further
comprises at least two snap-action hooks that are arranged on
opposite sides of the press-on region.
6. The heat sink of claim 1, wherein the press-on region includes a
quadrilateral geometry that matches the geometric shape of the rear
side of the surface-mounted semiconductor device and the heat sink
comprises four snap-action hooks.
7. The heat sink of claim 1, wherein the plastically deformable
composition comprises a silicone elastomer filled with aluminum
oxide and boron nitride.
8. The heat sink of claim 1, wherein the plastically deformable
composition comprises a plastic mixture with plasticizer additions
and thermally conductive metal particles.
9. The heat sink according to claim 1, wherein the plastically
deformable composition comprises a metal paste.
10. The heat sink of claim 9, wherein the metal paste comprises an
aluminum paste.
11. The heat sink of claim 1, wherein the heat sink consists of a
single piece including the press-on region and snap-action
hooks.
12. The heat sink of claim 1, wherein the snap-hooks comprise limbs
including stop corners that limit the engagement of the snap-action
hooks with the passage openings of the circuit board.
13. The heat sink of claim 1, wherein the heat sink includes stop
brackets extending from portions of the heat sink and configured to
limit tilting of the heat sink when the snap-action hooks are
latched into corresponding passage openings of the circuit
board.
14. The heat sink of claim 1, wherein the force generated by the
heat sink upon the snap-action hooks being latched into
corresponding passage openings of the circuit board effects the
deformation of the plastically deformable thermally conductive
composition to form the intermediate layer.
15. A semiconductor device mounted on a circuit board, wherein the
semiconductor device comprises the heat sink of claim 1.
16. The semiconductor device of claim 15, wherein the semiconductor
device includes a circuit substrate with surface-mountable contacts
disposed on an underside of the semiconductor device and a
semiconductor chip with flip-chip contacts disposed on a top side
of the semiconductor device, the surface-mountable contacts of the
circuit substrate including soldering connections to contact pads
of the circuit board.
17. A semiconductor module on a circuit board, wherein the
semiconductor module comprises the heat sink of claim 1.
18. A semiconductor system comprising: a circuit board; a
semiconductor device mounted on the circuit board; and a heat sink
comprising a three-dimensionally structured thermally conductive
plate with a press-on region and with snap-action hooks, the
snap-action hooks being resiliently connected to the press-on
region of the heat sink and being configured for latching into
place, with pressure generation of the press-on region onto a rear
side of the semiconductor device, into corresponding passage
openings of the circuit board, and a plastically deformable
thermally conductive composition disposed on the heat sink and
being arranged to form an intermediate layer between the rear side
of the semiconductor device and the press-on region of the heat
sink.
19. A method for mounting a heat sink on a circuit board including
at least one semiconductor device, comprising: positioning the
semiconductor device with a circuit substrate on the circuit board,
the circuit board including passage openings; applying a
plastically deformable composition to the heat sink in a press-on
region of the heat sink, the heat sink including snap-action hooks
that are resiliently connected to the press-on region; aligning the
snap-action hooks of the heat sink with the passage openings of the
circuit board; latching the snap-action hooks into the passage
openings so as to deform the plastically deformable composition,
the deformed composition forming an intermediate layer between the
heat sink and a rear side of the semiconductor device.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C. .sctn.119
to German Application No. DE 10 2005 002 812.8, filed on Jan. 20,
2005, and titled "Heat Sink for Surface-Mounted Semiconductor
Devices and Mounting Method," the entire contents of which are
hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] The invention relates to a heat sink for surface-mounted
semiconductor devices on a main circuit board of an electronic
module, and a mounting method for the same.
BACKGROUND
[0003] U.S. Pat. No. 6,233,150 discloses a memory module having a
memory card including a circuit board and a number of electronic
components mounted on the circuit board. The memory module has, as
a heat sink, a pair of covers arranged on mutually opposite
surfaces of the circuit board and a pair of clamps that press the
two covers onto the circuit board. In this case, each of the covers
is thermally and electrically conductive and affords protection
from electromagnetic radiation.
[0004] A complete heat sink of this type extends over all
semiconductor devices of a memory module and thus thermally couples
all of the semiconductor devices independently of their different
power losses in a common housing. One disadvantage to this design
is impermissible heating of adjacent semiconductor devices. A
further disadvantage is the high material outlay, which thus
increases the costs. Yet another disadvantage is that, in addition
to producing the covers, it is also necessary to produce clamping
elements in the form of clips, which increases the manufacturing
costs.
[0005] U.S. Pat. No. 6,188,576 discloses a memory module having a
housing cover in order to enclose a circuit board which has a
plurality of individual semiconductor devices that dynamically
generate a different quantity of heat. The housing cover provides a
heat dissipation for the plurality of different memory chips. The
different memory chips are thus thermally interconnected among one
another via the circuit board and via corresponding solder balls.
Besides the heat dissipation, the rigid housing protects both the
circuit board and the chips.
[0006] In an embodiment disclosed in U.S. Pat. No. 6,188,576, a
memory module includes a thermally conductive substance arranged
within the housing cover in order to conduct heat from the
individual chips to the housing cover. In this case, the covers
have snap-action hooks which reach over the edges of the circuit
board and clamp the covers onto the circuit board on both sides. In
this design, there is the risk of impermissible heating of adjacent
semiconductor devices which intrinsically generate a reduced power
loss and are then impermissibly additionally heated by the heat
distribution of the thermally conductive substance and the
thermally conductive covers. Moreover, there is also the
disadvantage of an increased material outlay, which increases
manufacturing costs.
[0007] A further known heat sink design includes providing
individual semiconductor devices directly with a heat sink, where
the heat sink is adhesively bonded onto the rear side of the
housing by means of a thermally conductive adhesive. Solutions of
this type have the disadvantage that, in mechanical shock tests and
in vibration tests and also in the case of other manual influences,
an impermissibly high mechanical stress may be exerted on the
devices to be cooled by virtue of the heat sink bonded on
adhesively, which may result in damage to the semiconductor
device.
SUMMARY OF THE INVENTION
[0008] It is an object of the invention to provide a heat sink for
a semiconductor device that enables cooling of a product with a
flip-chip semiconductor housing in accordance with a BGA (ball grid
array) type with an open chip rear side, the semiconductor chip
being mounted on the substrate of a memory module by its flip-chip
contacts.
[0009] It is another object of the invention to provide such a heat
sink for a semiconductor device in which the memory module is a
DIMM (dual inline memory module) semiconductor device.
[0010] It is a further object of the invention to provide such a
heat sink that permits the flip-chip housing to be hotter than the
surrounding components in an operating state.
[0011] It is still another object of the invention to provide such
a heat sink such that only individual semiconductor devices in the
form of flip-chip housings are cooled by the heat sink, and where
further components such as, for example, DRAMs, are not heated (or
only heated to a reduced extent) by the heat generated by the
flip-chip housings.
[0012] The aforesaid objects are achieved individually and/or in
combination, and it is not intended that the present invention be
construed as requiring two or more of the objects to be combined
unless expressly required by the claims attached hereto.
[0013] In accordance with one embodiment of the present invention,
a heat sink is provided for surface-mounted semiconductor devices
on a main circuit board of an electronic module, preferably a
modular memory device. The heat sink includes a three-dimensionally
structured thermally conductive plate with a press-on region and
with snap-action hooks. The snap-action hooks are arranged
approximately at right angles with respect to the press-on region
and are spring-elastically connected to the press-on region of the
heat sink. The snap-action hooks are latched into place in passage
openings of the circuit board with pressure generation of the
press-on region onto a rear side of the surface-mounted
semiconductor device. A plastically deformable, thermally
conductive composition is arranged between the rear side of the
semiconductor device and the press-on region of the heat sink as an
intermediate layer.
[0014] In accordance with another embodiment of the invention, a
mounting method for fitting a heat sink on a circuit board with at
least one semiconductor device includes the following method steps.
A surface-mountable semiconductor device with circuit substrate is
oriented and positioned on the main circuit board with passage
opening for snap-action hooks of the heat sink. In preparation, the
plastically deformable composition is applied to be heat sink in
its press-on region. A heat sink is subsequently taken up from a
heat sink supply tray, the heat sink including snap-action hooks at
right angles with respect to the press-on region, which, for its
part, is spring-elastically connected to the snap-action hooks. The
snap-action hooks of the heat sink are then aligned with the
passage openings of the circuit board. Afterward, by exercising
pressure, the snap-action hooks are latched into place with
deformation of the ductile thermally conductive composition to form
an intermediate layer between the heat sink and the rear side of
the semiconductor device.
[0015] The above and still further objects, features and advantages
of the present invention will become apparent upon consideration of
the following detailed description of specific embodiments thereof,
particularly when taken in conjunction with the accompanying
drawings wherein like reference numerals in the various figures are
utilized to designate like components.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 shows a schematic cross section through a heat sink
of one embodiment of the invention.
[0017] FIG. 2 shows a schematic plan view of the heat sink of FIG.
1.
[0018] FIG. 3 shows a schematic cross section through a
semiconductor device connected with the heat sink of FIG. 1, where
the heat sink deforms a ductile thermally conductive material upon
placement onto a circuit board of the semiconductor device.
[0019] FIG. 4 shows a schematic cross section through the heat sink
with spring-elastic deformation when the snap-action hooks are
introduced into passage openings of the circuit board.
[0020] FIG. 5 shows a schematic cross section through the heat sink
of FIG. 4 after the snap-action hooks have been anchored.
[0021] FIG. 6 shows a schematic partial view in perspective of a
heat sink with two snap-action hooks, one of the snap-action hooks
being shown prior to introduction into a passage opening.
[0022] FIG. 7 shows a schematic bottom view of the heat sink of
FIG. 1 with a ductile thermally conductive composition having been
applied.
[0023] FIG. 8 shows a schematic partial view in perspective of the
underside of a circuit board after a snap-action hook has been
locked.
[0024] FIG. 9 shows a schematic partial side view of the heat sink
with locked snap-action hook and stop bracket.
[0025] FIG. 10 shows a partial view in perspective of the rear side
of a semiconductor chip with a deformed thermally conductive
ductile composition after removal of the heat sink.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] In accordance with the invention, a heat sink is provided
for surface-mounted semiconductor devices on a main circuit board
of an electronic module, preferably a modular memory device. The
heat sink includes a three-dimensionally structured thermally
conductive plate with a press-on region and with snap-action hooks.
The snap-action hooks are arranged approximately at right angles
with respect to the press-on region and are spring-elastically
connected to the press-on region of the heat sink. The snap-action
hooks are latched into place in passage openings of the circuit
board with pressure generation of the press-on region onto a rear
side of the surface-mounted semiconductor device. A plastically
deformable, thermally conductive composition is arranged between
the rear side of the semiconductor device and the press-on region
of the heat sink as an intermediate layer.
[0027] The following advantages are achieved by the both thermally
and mechanically calculated and simulated design of the heat sink
and by the design of the plastically deformable and thermally
conductive composition:
[0028] 1. The tolerances of the semiconductor device and the
tolerances of the circuit board are largely compensated for by the
embodiment of the spring-elastic connections of the heat sink and
by the design and the properties of the plastically deformable and
thermally conductive composition. This means that, given maximum
dimensions with regard to the thickness of the semiconductor device
to be cooled and the dimensions of the circuit board, as a result
of the then increased spring force of the elastic connections of
the heat sink, the plastically deformable and thermally conductive
composition forms a thinner intermediate layer between press-on
region and snap-action hooks. Given minimum dimensions, the ductile
thermally conductive composition then behaves in the opposite
fashion. This means that the plastically deformable and thermally
conductive composition forms a thicker intermediate layer
corresponding to the tolerances.
[0029] 2. In the mounted state, the heat sink permits forced
cooling air to contribute to the cooling on the entire surface of
the heat sink and, in particular, also in regions of the underside
of the heat sink. For the underside cooling, the spring-elastic
connections to the snap-action hooks are formed in correspondingly
planar fashion.
[0030] 3. The heat sink is pressed and deformed in a defined manner
in the course of mounting in order to introduce the snap-action
hooks into the circuit board or into the passage openings in the
circuit board. After mounting and the now released spring force of
the elastic connection between press-on region and snap-action
hooks, the press-on region holds securely on the semiconductor
device housing and on the circuit board. In this case, the
plastically deformable and thermally conductive composition is
compressed to a thickness of the intermediate layer that
compensates for the tolerances.
[0031] 4. The snap-action hooks that reach through the fixing
openings or passage openings of the circuit board anchor the heat
sink reliably and securely. In this case, it has been shown that a
fixed retention and no deformation of the heat sink result in shock
tests up to 1500 g and vibration tests.
[0032] 5. Utilizing support parts such as stop brackets on the heat
sink, the latter can be spring-elastically prestressed only in a
specific permissible region, so that the semiconductor device and
the associated semiconductor chip are not damaged.
[0033] 6. By virtue of an optimized design of the snap-action
hooks, only a minimal fixing area is required in the circuit board
so that the area for the wiring on the circuit board is only
minimally reduced.
[0034] 7. Since the heat sink constitutes a three-dimensionally
structured thermally conductive plate, it is suitable for mass
production and embodied in machine-mountable fashion.
[0035] 8. The heat sink is easy to remove on account of the
snap-action hooks and permits repair of the underlying
semiconductor devices.
[0036] 9. The heat sink enables repair of adjacent semiconductor
devices, such as DRAMs, without removing the heat sink itself.
[0037] In one preferred embodiment of the invention, the plasticity
of the plastically deformable composition is matched to the
pressure in the press-on region for the elasticity of a
spring-elastic connection between press-on region and snap-action
hooks. This mechanically calculated and simulated construction
makes it possible, as mentioned above, to enable compensation of
minimum tolerances of the semiconductor devices in terms of their
thickness by means of the intermediate layer and thus by means of
the plastically deformable composition.
[0038] In a further embodiment of the invention, the passage
openings through the circuit board are metal-coated. This has the
advantage that, via the metal coating of the passage openings, the
heat sink can be electrically connected to a ground potential and
thus additionally has a shielding effect with respect to
electromagnetic interference radiation.
[0039] In a further preferred embodiment of the invention, the heat
sink has at least two snap-action hooks which are arranged in a
manner situated opposite with respect to the press-on region. The
planar compensation by the two snap-action hooks is achieved by the
planar extent of the three-dimensionally structured thermally
conductive plate from which the heat sink is formed.
[0040] In a further embodiment of the invention, the press-on
region is quadrilated and is matched to the shape of the rear side
of a surface-mounted semiconductor device, so that it preferably
has four snap-action hooks, i.e. in each case one at each corner.
In this embodiment of the invention, the press-on effect of the
snap-action hooks is equalized, and helps to compensate for
tolerances in the device thickness. In order to achieve a
dimensionally stable and rigid press-on region, the
three-dimensionally structured plate has an offset between the
spring-elastic region of the heat sink and the press-on region. In
this case, the offset forms the edges of the press-on region and
thus provides for the dimensionally stable contour of the press-on
region with respect to the spring-elastic structure of the
plate-type heat sink, which extends from the press-on region as far
as the snap-action hooks angled away at right angles from the
press-on region.
[0041] The plastically deformable, readily thermally conductive
composition preferably includes a silicone elastomer filled with
aluminum oxide or boron nitride. In this case, the silicone
elastomer undertakes the plastically deformable function while the
thermal conduction is effected or improved by means of the aluminum
oxide and boron nitride. In one preferred embodiment of the
invention, the heat sink itself may include thermally conductive
metal, such, as, for example, a copper alloy in the form of
CuCrSiTi (Cr 0.3% by weight, Ti 0.1% by weight, Si 0.02% by weight,
remainder copper with a thermal conductivity of 310 W/mK).
[0042] The metal used for the three-dimensionally structured
thermally conductive plate used as a heat sink includes
corresponding mechanical properties in order, on the one hand, to
ensure the spring properties in the spring-elastic region, and, on
the other hand, to have a corresponding dimensional stability in
the press-on region. The mechanical properties preferably have a
tensile strength >530 MPa and a 0.2% proof stress >460 MPa.
The thermal contact area of the heat sink is formed by the
abovementioned press-on region, which bears, after the mounting of
the heat sink, on the rear side of the semiconductor device with
anchoring of the snap-action hooks fixedly on the semiconductor
device or on the intermediate layer made of plastically deformable
thermally conductive material.
[0043] In an alternative embodiment of the invention, the
plastically deformable composition is a metal alloy including
metals having a low melting point, preferable indium, tin and/or
lead. This mechanic composition has the advantage that it not only
exhibits good thermal conductivity, but also constitutes a good
connection electrically if the rear side of the semiconductor
device is to be grounded.
[0044] In a further embodiment of the invention, the plastically
deformable composition includes a plastic mixture with plasticizer
additions and thermally conductive metal particles. In this case,
the plasticizer additions ensure that the plastic becomes
plastically deformable and thus ductile, and the conductive metal
particles advantageously provide for the good thermal properties.
Finally, it is also possible for the plastically deformable
composition used to be a tough viscous metal paste, preferably an
aluminum paste, which, on the one hand, is adapted to the
tolerances of the semiconductor devices even at low pressure and,
on the other hand, forms an ideal intermediate layer with thermally
conductive properties.
[0045] In a further embodiment of the invention, the heat sink is
constructed in one piece with press-on region and snap-action
hooks. This has the advantage that the complete heat sink with
press-on region, spring-elastic connecting region and snap-action
hooks can be stamped out in one piece from a corresponding metal
plate by means of a single stamping process.
[0046] In a further embodiment of the invention, the snap-action
hooks have limbs which are reinforced by a stiffening bead and have
stop corners that delimit the engagement region of the snap-action
hooks. The stop corners are likewise arranged approximately at
right angles on the limbs and are supported on the top side of the
circuit board when the snap-action hooks are introduced into the
opening of the circuit board, and thus limit the force which acts
on the deformable thermally conductive composition between heat
sink and semiconductor device.
[0047] In a further embodiment of the invention, the heat sink has
stop brackets. This has the advantage that excessive tilting of the
heat sink is thereby prevented and the semiconductor device or the
semiconductor chip is not damaged by shock, vibration or incorrect
handling.
[0048] In a further specific embodiment of the invention, the
semiconductor device has a circuit substrate with surface-mountable
contacts on its underside and a semiconductor chip with
chip-contacts on its top side (i.e., a side opposing the
underside). In this case, the snap-action hooks are in engagement
with the openings of the circuit board. In this embodiment of the
invention, due to the press-on region of the heat sink and the
ductile composition arranged in between, a pressure is implemented
on the rear side of the semiconductor chip and a regular
distribution of the thermally conductive composition is achieved on
the rear side of the semiconductor device. The magnitude of the
pressure or of the force on the deformable composition depends on
the spring elasticity of the connecting region of the heat sink and
may be dimensioned in such a way as to obtain an optimum thermal
coupling of heat sink and semiconductor device taking account of
the tolerances of the components of the semiconductor device
involved and simultaneous deformation of the ductile thermally
conductive composition.
[0049] While it is possible, on the one hand, for the external
contacts of the semiconductor device to be surface-mounted on the
main circuit board by soldering connections, in a further aspect of
the invention it is possible to coordinate the spring force of the
heat sink and the deformability of the ductile composition with one
another in such a way that the surface-mountable contacts on the
underside of the circuit substrate of the semiconductor device form
a pressure contact or a pressure contact connection to
corresponding contact pads of the circuit board. Consequently, it
emerges in a further embodiment of the invention that the
surface-mountable contacts of the circuit substrate have pressure
contact connections to contact pads of the main circuit board, the
contact pressure being applied to the surface-mountable contacts by
the stop brackets and the press-on region.
[0050] The invention relates not only to the heat sink but also to
semiconductor devices which are provided with a heat sink of this
type. Furthermore, the invention also relates to semiconductor
modules, in particular memory modules in DIMM technology (dual
inline memory module technology), which may have individual or a
plurality of heat sinks of this type.
[0051] A mounting method for fitting a heat sink on a circuit board
with at least one semiconductor device includes the following
method steps. Firstly, a surface-mountable semiconductor device
with circuit substrate is oriented and positioned on the main
circuit board with passage opening for snap-action hooks of the
heat sink. In preparation, the plastically deformable composition
is applied to be heat sink in its press-on region. A heat sink is
subsequently taken up from a heat sink supply tray, the heat sink
having snap-action hooks at right angles with respect to the
press-on region, which, for its part, is spring-elastically
connected to the snap-action hooks. The snap-action hooks of the
heat sink are then aligned with the passage openings of the circuit
board.
[0052] Afterward, by exercising pressure, the snap-action hooks are
latched into place with deformation of the ductile thermally
conductive composition to form an intermediate layer between the
heat sink and the rear side of the semiconductor device.
[0053] This method has the advantage that, with few automatic
handling processes, the heat sink can be arranged and anchored on
corresponding positions in a semiconductor memory module and, at
the same time, an intensive thermal coupling between heat sink and
semiconductor device can be produced by means of the ductile
thermally conductive composition. One variant of the method has the
advantage that soldering the surface contacts of the semiconductor
device or of the circuit substrate onto the circuit board may
possibly be dispensed with if the contact pressure exerted on the
press-on region of the heat sink suffices to fix the
surface-mountable contacts in their position on the contact pads of
the circuit board by means of the forces of the anchored
snap-action hooks and the connecting region between snap-action
hooks and press-on region. However provision is primarily made for
soldering the surface-mountable contacts on the contact pads of the
circuit board, so that extreme loads can be transmitted.
Furthermore, it is possible for the surface-mountable contacts to
be adhesively bonded onto the contact pads of the circuit board
using a conductive adhesive.
[0054] The abovementioned pressure contact connection has the
further advantage over soldering or adhesively bonding the
surface-mountable contacts in place that fewer shear stresses act
on the surface-mountable contacts during cyclic thermal loading.
The reliability of the semiconductor devices is thus increased by
virtue of this mounting on the circuit board.
[0055] The effectiveness of the novel heat sink for memory modules
was verified with the aid of simulation methods and wind tunnel
experiments. TABLE-US-00001 TABLE 1 T.sub.ambient = 55.degree. C.
Thickness: 0.3 mm Thickness: 0.5 mm V.sub.air = 1/5 m/s .lamda. =
120 W/mK .lamda. = 243 W/mK .lamda. = 310 W/mK .lamda. = 120 W/mK
.lamda. = 243 W/mK .lamda. = 310 W/mK Area of the laminar T.sub.max
103 100 99 100 98 97 semi- (AMB) conductor T.sub.max 96 96 96 96 96
96 chip: (DRAM) 62.25 mm.sup.3 turbulent T.sub.max 98 95 94 95 93
92 (AMB) T.sub.max 88 88 88 88 88 88 (DRAM)
Table 1 shows the results of a thermal simulation during which a
heat sink having an overall height of 3.49 mm was tested. The test
was carried out at an ambient temperature T.sub.ambient=55.degree.
C. and at an air velocity V.sub.air=1.5 m/s in a wind tunnel. The
area of the rear side of the semiconductor chip on which the heat
sink is pressed with its press-on region by means of an
intermediate layer made of a plastically deformable and thermally
conductive composition is 62.25 mm.sup.2. The tests were carried
out using three-dimensionally structured plates as heat sinks
having a thickness of 0.3 mm and, in comparison therewith, a
thickness of 0.5 mm.
[0056] The maximum temperatures at the AMB devices provided with a
heat sink and the maximum temperatures at the DRAMs not equipped
with cooling areas are measured for different thermal
conductivities .lamda. of the cooling materials. The thermal
conductivities were varied in three stages with .lamda.=120 W/mK,
243 W/mK and 310 W/mK. It can be seen in this case that the maximum
temperature of the AMB devices provided with heat sinks decreases
as the thermal conductivity increases, to be precise from
103.degree. C. to 99.degree. C. The heat sinks comprising a wall
thickness of 0.5 mm result in correspondingly lower maximum
temperatures for the AMB devices of between 100 and 97.degree.
C.
[0057] The DRAMs without a heat sink were able to be kept constant
at 96.degree. C. during laminar air flow and constant at 88.degree.
C. during turbulent flow even though the AMB devices with their
increased power losses are arranged adjacent to the DRAMs on a
circuit board.
[0058] If the ambient temperature is increased by a few degrees to
60.degree. C. and if, at the same time, a thickness between the two
thicknesses specified in the table, of 0.4 mm, is assumed, a
maximum temperature of the AMB devices of 98.degree. C. results in
the simulation, given a highest thermal conductivity value of 310
W/mK and turbulent flow.
[0059] It could thus be shown that an effective cooling by the heat
sinks of individual semiconductor devices is possible without the
temperature of the adjacent DRAM devices being adversely influenced
thereby.
[0060] Exemplary embodiments of heat sinks incorporated with
semiconductor devices in accordance with the invention are
illustrated in FIGS. 1-10.
[0061] FIG. 1 shows a schematic cross section through a heat sink 1
of one embodiment of the invention. The heat sink 1 is shown in
profile here and has a press-on region 5 at its center and two
spring-elastic connecting regions 11 on each side of the press-on
region 5. In this embodiment of the invention, the press-on region
5 has a well contour with a plane bottom 22 and raised-up edges 23
and 24. These edges 23 and 24 merge with the connecting regions 11,
at which snap-action hooks 6 are arranged virtually
perpendicularly. Snap-action hooks 6 can engage in passage openings
of a circuit board (not shown) and be anchored on the circuit
board, so that the press-on region 5 can be pressed by its bottom
22 onto the rear side of a semiconductor device (not shown) and be
fixed there. Moreover, stop brackets 12 are arranged in a manner
angled away perpendicularly with respect to the connecting regions
11, and prevent excessive tilting of the heat sink.
[0062] FIG. 2 shows a schematic plan view of the heat sink 1 of
FIG. 1. This plan view reveals that two snap-action hooks 6 are
arranged relatively centrally on mutually opposite sides 25 and 26
of the heat sink 1. The dashed-dotted line 27 shows the outer
contour of the semiconductor device 2 having a circuit substrate
13, on which a semiconductor chip 16 is situated, the edges 30 of
which are identified by a dashed-dotted line 28. The contours of
the semiconductor chip 16 lie within the bottom 22 of the press-on
region 5 of the heat sink 1. The heat sink 1 includes, in the
spring-elastic connecting regions 11, four stop brackets 12 in the
four corners 37, 38, 39 and 40, in order to limit tilting of the
heat sink and in order to avoid damage to the semiconductor
chip.
[0063] At the center of the heat sink 1, the press-on region 5
exerts a pressure on the plastically deformable composition (not
visible in FIG. 2), which propagates under pressure over the whole
area on the rear side of a semiconductor device and produces a good
thermal transition from the heat sink 1 in the press-on region 5 to
the semiconductor chip. The heat sink 1 has slots 29 at the edges
of the press-on region, and the slots render a compliant transition
from the press-on region 5 to the spring-elastic connecting region
11.
[0064] FIG. 3 shows a schematic cross section through a
semiconductor device 2, which is pressed onto a circuit board 3 by
the heat sink 1 of FIG. 1. The semiconductor device 3 has a BGA
construction. In the embodiment of FIG. 3, the semiconductor device
includes a circuit substrate 13, a wiring structure 41 disposed on
the top side 18 of the substrate 13, and surface-mountable contacts
14 on the underside 15 of the substrate 13. The surface-mountable
contacts 14 on the underside 15 are electrically connected to the
wiring structure 41 on the top side 18 of the circuit substrate 13.
A semiconductor chip 16 is arranged on the top side 18 of the
circuit substrate 13, and is electrically connected via flip-chip
contacts 17 to the wiring structure 41 on the top side 18 of the
wiring substrate 13.
[0065] A contact pressure of the press-on region 5 acts on the rear
side 7 of the semiconductor chip 16 in arrow direction A. A
plastically deformable thermally conductive composition 9 is
arranged between the press-on region 5 of the heat sink 1 and the
rear side 7 of the semiconductor chip 16, which composition forms
an intermediate layer 10 and compensates for tolerances in the
thickness of the semiconductor chip 16 and also in the thickness of
the circuit substrate 13 and also in the height of the
surface-mounted contacts 14. Moreover, the intermediate layer 10
ensures the thermal contact between the rear side 7 of the
semiconductor chip 16 and the heat sink 1. In this embodiment of
the invention, this ductile composition 9 includes a silicone
elastomer filled with aluminum oxide and boron nitride. In this
case, the aluminum oxide and the boron nitride provide for a good
thermal conductivity and the silicone elastomer provides for the
ductility of this composition of the intermediate layer 10.
[0066] The heat sink 1 is a three-dimensionally structured
thermally conductive plate 4 that includes, in addition to the
press-on region 5, connecting regions 11 on both sides of the
press-on region 5. The connection regions 11 are spring-elastic and
merge with snap-action hooks 6 arranged generally vertically with
respect to the connecting region 11 and with respect to the
press-on region 5. The snap-action hooks 6 are arranged only on
short side sections of the heat sink 1. They extend through passage
openings 8 of the circuit board 3 on the top side 31 of which the
semiconductor device 2 is arranged, and are in engagement with the
underside 21 of the circuit board 3. The circuit board 3 has a
printed circuit for a semiconductor module. The remaining devices
of the semiconductor module are not shown here. However, these
remaining devices may or may not include a heat sink of the type
depicted in FIG. 3. In particular, in cases where the power loss of
a remaining component is not very high (e.g., as is the case for
DRAMs), a heat sink 1 of this type is omitted for such remaining
component.
[0067] The cross section through the mounted heat sink 1 on the
circuit board 3 shows that, in the case of a moving cooling medium,
such as air, cooling medium actively sweeps around both the
underside 32 and the top side 33 of the heat sink 1. In this
embodiment of the invention, the material of the heat sink 1
includes a readily conductive copper alloy CuCrSiTi (including Cr
0.3% by weight, Ti 0.1% by weight, Si 0.02% by weight and the
remainder copper). This copper alloy has a thermal conductivity of
310 W/mK, and has a tensile strength that is greater than 530 MPa.
The 0.2% proof stress lies above 460 MPa. These mechanical
properties with regard to the tensile strength and the proof stress
make it possible also to form the connecting regions 11 with their
spring-elastic properties from a sheet-metal plate of this
material, where a plate thickness of between 0.3 and 0.5 mm is
preferable. The intermediate layer 10 made of a ductile thermally
conductive material 9 makes it possible to provide a thermal
contact resistance of approximately 10 to 11 K/W between the rear
side 7 of the semiconductor chip 16 and the ambient air of the heat
sink 1.
[0068] FIG. 4 shows a schematic cross section through the heat sink
1 with spring-elastic deformation when the snap-action hooks 6 are
introduced into the passage openings 8 of a circuit board 3. For
this purpose, the limbs 34 and 35 of the snap-action hooks 6 are
compressed in arrow direction B, so that they can be led through
the passage openings 8 of the circuit board 3. For this purpose,
the heat sink 1 is lowered in arrow direction A and, at the same
time, a pressure is exerted on the rear side 7 of the semiconductor
chip 16 and the semiconductor device 2 is pressed with its contacts
14 onto the top side 31 of the circuit board 3 with its contact
pads 19.
[0069] FIG. 5 shows a schematic cross section through the heat sink
1 after the snap-action hooks 6 have been anchored on the underside
21 of the circuit board 3. The passage openings 8 are metallized in
this embodiment of the invention, so that there is the possibility
of connecting the heat sink 1 to a ground potential with the aid of
the metal layer 42 of the passage openings 8. The spring force of
the connecting regions 11 provides for the contact pressure in
arrow direction A in the press-on region 5 of the heat sink 1 on
the semiconductor device 2. As a result of the contact pressure,
the plastically deformable composition 9 is spread out over the
whole area on the rear side 7 of the semiconductor chip 16 and
produces a good thermal transition from the heat sink 1 to the
semiconductor chip 16.
[0070] FIG. 6 shows a schematic perspective view of a heat sink 1
with two snap-action hooks 6, where one of the snap-action hooks 6
is depicted in FIG. 6 just prior to introduction into a passage
opening 8. The circuit substrate 13 of the semiconductor device 2
can furthermore be seen schematically. The limb 35 of the
snap-action hook 6 has a stiffening bead 44, which prevents warping
of the limb 35 in the event of shock or vibration loading.
Moreover, two stop corners 45 and 46 are provided on the limb 35,
and prevent, during the mounting of the heat sink 1, the limb from
entering too far into opening 8 in the circuit board 3 and from
deforming the thermally conductive ductile composition to an
excessively great extent in the process. The final thickness of the
ductile composition is set by the spring force present in the heat
sink.
[0071] FIG. 7 shows a schematic bottom view of the heat sink 1 of
FIG. 1 with a ductile thermally conductive composition 9 having
been applied. The composition 9 is applied to the underside 32 in
the positional region of the semiconductor chip before the heat
sink 1 is fixed on the circuit board (not shown) with the aid of
the snap-action hooks 6.
[0072] FIG. 8 shows a schematic partial perspective bottom view of
the underside 21 of the circuit board 3 after a snap-hook 6 has
been locked. The passage opening 8 includes a metal layer 42, so
that the metal of the snap-action hook 6 forms a pressure contact
with the metal layer 42 of the passage opening 8. Furthermore,
wiring structure 36 can be seen on the underside 21 of the circuit
board 3.
[0073] FIG. 9 shows a schematic partial side view of the heat sink
1 with a locked snap-action hook 6 in the circuit board 3 and stop
brackets 12. The stop brackets 12 prevent tilting of the heat sink
1 and damage to the semiconductor device.
[0074] FIG. 10 shows a partial view in perspective of the rear side
7 of a semiconductor chip 16 with a ductile intermediate layer 10
being applied. In order to show this illustration, the heat sink
with its press-on region has been removed. It can clearly be
discerned that the ductile thermally conductive composition 9
completely covers the rear side 7 of the semiconductor chip 16 as a
result of the exerted pressure of the heat sink. Tolerances in the
thickness variation both of the semiconductor chip 16 and of the
circuit substrate 13 and also in the height of the contacts 14 on
the underside 15 of the circuit substrate 13 are compensated for by
this ductile thermally conductive composition 9.
List of Reference Symbols
[0075] 1 Heat sink [0076] 2 Surface-mountable semiconductor device
[0077] 3 main circuit board [0078] 4 Structured plate [0079] 5
Press-on region [0080] 6 Snap-action hook [0081] 7 Rear side of the
semiconductor device [0082] 8 Passage opening in the circuit board
[0083] 9 Plastically deformable composition [0084] 10 Intermediate
layer [0085] 11 Resilient connection [0086] 12 Stop bracket [0087]
13 Circuit substrate [0088] 14 Surface-mountable contacts of the
circuit substrate [0089] 15 Underside of the circuit substrate
[0090] 16 Semiconductor chip [0091] 17 Flip-chip contacts [0092] 18
Top side of the circuit substrate [0093] 19 Contact pads of the
circuit board [0094] 20 Soldering contact [0095] 21 Underside of
the circuit board [0096] 22 Bottom of the heat sink [0097] 23
Raised-up edge [0098] 24 Raised-up edge [0099] 25 Side of the heat
sink [0100] 26 Side of the heat sink [0101] 27 Dash-dotted line
[0102] 28 Double-dotted line [0103] 29 Slot [0104] 30 Edges of the
semiconductor chip [0105] 31 Top side of the circuit board [0106]
32 Underside of the heat sink [0107] 33 Top side of the heat sink
[0108] 34 Limb of the snap-action hook with stop [0109] 35 Limb of
the snap-action hook with stop [0110] 36 Wiring structure [0111] 37
Corner of the circuit substrate [0112] 38 Corner of the circuit
substrate [0113] 39 Corner of the circuit substrate [0114] 40
Corner of the circuit substrate [0115] 41 Wiring structure on the
top side of the circuit substrate [0116] 42 Metal layer [0117] 44
Stiffening bead [0118] 45 Stop corners [0119] 46 Stop corners
[0120] A Arrow direction [0121] B Arrow direction
* * * * *